For the seminal publication, Traité pratique de gravure héliographique sur acier et sur verre (A Practical Treatise on Photogravure Engraving on Steel and on Glass) by Niépce de Saint-Victor (1805-1870), either he or his publisher commissioned a portrait for the frontispiece.
To further celebrate Niépce’s important discoveries in photographic printing, they made the portrait using his own process: “Gravure héliographique d’après une photographie sur acier selon les procédés de Niépce de Ste Victor” (Photogravure engraving after a photograph on steel, according to the methods of Niépce de Ste Victor).
Claude-Félix-Abel Niépce de Saint-Victor (1805-1870), Traité pratique de gravure héliographique sur acier et sur verre / par M. Niépce de Saint-Victor (Paris: Librairie de V. Masson, 1856). Provenance: C. F. Chandler. Graphic Arts Collection 2006-3213N
Note the photograph was taken by Victor Plumier (1821-1878) and a great deal was made about the fact that he did not retouch his negative. The engraving plate was made by Madame [is it Pauline?] Riffaut and the portrait finished by Adolphe-Pierre Riffaut (1821-1859).
If you use a microscope, you will see the hand engraving on top of the aquatint done, in particular, in the beard. In addition, there are gouache highlights delicately added to the paper print in the hair and the beard.